Rework srlinux testing helpers 🎄

Plan:

• Create pytest-inmanta-srlinux package (open source)
• Have a central class representing a device
• Add one method to compile and push a model to the device
  • Pass as argument the parameters for the model
  • Load model parameters using std::getenv (+ pytest monkeypatch to set env var?)
  • Pack some basic models in the library
• Add methods to the class to support actions on the device (ssh or api)
  • Ping
  • Get routing table
• Each method should compile a model (filling in parameters, via env vars? or template?) then deploy the yang resource (single resource in the model)
• Import helpers from pytest-inmanta-connect and athonet-mpn test suite

💡 Ideas:

• Update topologies to add cpe to each router
• Configure cpe in the model (should be possible to enable/disable)
• In tests, when cpe is not part of the service, import the model and export to configure it anyway
• Ping from cpe to cpe
• Start with connect, move as much as possible to cpe